Historical Type Ia supernovae are a leading candidate for the source of
positrons observed through their diffuse annihilation emission in the Galaxy.
However, search for annihilation emission from individual Type Ia supernovae
has not been possible before the improved sensitivity of \integral. The total
511 keV annihilation flux from individual SNe Ia, as well as their contribution
to the overall diffuse emission, depends critically on the escape fraction of
positrons produced in $^{56}$Co decays. Late optical light curves suggest that
this fraction may be as high as 5%. We searched for positron annihilation
radiation from the historical Type Ia supernova SN 1006 using the SPI
instrument on \integral. We did not detect significant 511 keV line emission,
with a 3$\sigma$ flux upper limit of 0.59 x 10$^{-4}$ ergs cm^-2 s^-1 for \wsim
1 Msec exposure time, assuming a FWHM of 2.5 keV. This upper limit corresponds
to a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and
rules out the possibility that Type Ia supernovae produce all of the positrons
in the Galaxy (~ 12% escape fraction), if the mean positron lifetime is less
than 10$^{5}$ years. Future observations with \integral will provide stronger
limits on the escape fraction of positrons, the mean positron lifetime, and the
contribution of Type Ia supernovae to the overall positron content of the
Galaxy.Comment: 3 pages, 2 figures, accepted for publication in ApJ

E. Kalemci

Milne D. K.

P. A. Milne

S. E. Boggs

S. P. Reynolds

English

arXiv

Historical Type Ia supernovae are a leading candidate as the source of positrons observed through diffuse annihilation emission in the Galaxy. However, a search for annihilation emission from individual Type Ia supernovae was not possible before the improved sensitivity of INTEGRAL. The total 511 keV annihilation flux from individual SNe Ia, as well as their contribution to the overall diffuse emission, depends critically on the escape fraction of positrons produced in 56Co decays. Late optical light curves suggest that this fraction may be as high as 5%. We have searched for positron annihilation radiation from the historical Type Ia supernova SN 1006 using the SPI instrument on INTEGRAL. We did not detect significant 511 keV line emission, with a 3 σ flux upper limit of 0.59 ×10-4 photons cm-2 s-1 for ~1 Ms exposure time, assuming a FWHM of 2.5 keV. This upper limit corresponds to a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and rules out the possibility that Type Ia supernovae produce all of the positrons in the Galaxy (~12% escape fraction), if the mean positron lifetime is less than 105 yr. Future observations with INTEGRAL will provide stronger limits on the escape fraction of positrons, the mean positron lifetime, and the contribution of Type Ia supernovae to the overall positron content of the Galaxy

Kalemci, Emrah

Boggs, Steven E.

Milne, Peter A.

Reynolds, Stephen

Sabanci University Research Database

L55The Astrophysical Journal, 640:L55–L57, 2006 March 20 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.SEARCHING FOR ANNIHILATION RADIATION FROM SN 1006 WITH SPI ON INTEGRALE. Kalemci,1,2 S. E. Boggs,1,3 P. A. Milne,4 and S. P. Reynolds5˙Received 2005 July 25; accepted 2006 February 9; published 2006 March 1ABSTRACTHistorical Type Ia supernovae are a leading candidate as the source of positrons observed through diffuse annihi-lation emission in the Galaxy. However, a search for annihilation emission from individual Type Ia supernovaewas not possible before the improved sensitivity of INTEGRAL. The total 511 keV annihilation flux from individ-ual SNe Ia, as well as their contribution to the overall diffuse emission, depends critically on the escape fraction ofpositrons produced in 56Co decays. Late optical light curves suggest that this fraction may be as high as 5%. Wehave searched for positron annihilation radiation from the historical Type Ia supernova SN 1006 using the SPIinstrument on INTEGRAL. We did not detect significant 511 keV line emission, with a 3 j flux upper limit of 0.59#104 photons cm2 s1 for ∼1 Ms exposure time, assuming a FWHM of 2.5 keV. This upper limit correspondsto a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and rules out the possibility that TypeIa supernovae produce all of the positrons in the Galaxy (∼12% escape fraction), if the mean positron lifetime isless than 105 yr. Future observations with INTEGRAL will provide stronger limits on the escape fraction of positrons,the mean positron lifetime, and the contribution of Type Ia supernovae to the overall positron content of the Galaxy.Subject headings: gamma rays: observations — ISM: individual (SN 1006) — supernova remnants1. INTRODUCTIONSearching for g-ray lines from supernovae (SNe) remainsone of the primary goals of g-ray astrophysics, and also theInternational Gamma-Ray Astrophysics Laboratory (INTE-GRAL; Winkler et al. 2003a), as g-ray line studies can probethe nucleosynthesis and explosion kinematics of these events.Many of the nuclear decay chains (56Co, 44Ti) that produceg-ray lines in young SNe also produce positrons, making thema prime candidate as the source of positrons for the diffuseGalactic annihilation emission. However, there has not been adetection of positron annihilation radiation from an individualSN or a SN remnant (SNR). The main uncertainties on theexpected annihilation fluxes from young SNe are the meanlifetime and escape fraction of positrons. For Type Ia SNe, thelifetime of a positron will be small in the initial SN but maybecome as high as 104 years or more as the SN expands. Themean positron lifetime will be even longer, over 105 years, incases where thermalization takes place in the interstellar me-dium (ISM) rather than in the ejecta (Guessoum et al. 1991).If the ejecta’s magnetic field is weak or radially combed, then95% of the 56Co decay positrons annihilate promptly, and 5%escape to annihilate on a longer timescale (Chan & Lingenfelter1993). Alternatively, a tangled and strong magnetic field wouldconfine ∼100% of the 56Co positrons, resulting in prompt an-nihilation and leaving a much smaller annihilation radiationflux from the delayed 44Ti decays (t ∼ 85 yr).Simulations of late optical B and V light curves of Type IaSNe indicate that the 5% escape of the 56Co positrons is themore probable scenario, assuming the optical light curves tracethe bolometric luminosity (Milne et al. 1999). A later studyalso showed that 16 of 22 Type Ia SNe observed at late epochs1 Space Sciences Laboratory, University of California at Berkeley, 7 GaussWay, Berkeley, CA 94720-7450.2 Current address: Sabancı U¨ niversitesi, Orhanlı-Tuzla, 34956 I˙stanbul,Turkey.3 Department of Physics, University of California at Berkeley, 366 Le ConteHall, Berkeley, CA 94720-7300.4 Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721.5 Department of Physics, North Carolina State University, 2700 StinsonDrive, Box 8202, Raleigh, NC 27695.exhibit BVRI light curves of the same shape as the initial sam-pling of 10 SNe Ia, strengthening the conclusion of a 5% escapefraction (Milne et al. 2001b). However, excess near-infrared(NIR) emission at late epochs was reported recently from SN1998bu and SN 2000cx (Spyromilio et al. 2004; Sollerman etal. 2004). This excess may be due to emission shifting into theNIR wavelength range from the optical range. The optical-NIRbolometric light curves for those SNe favor full positron trap-ping. Observations of positron annihilation radiation from in-dividual SNRs can improve our understanding of the magneticfield configurations in these objects and also determine theirrole in producing the diffuse Galactic annihilation emission.The positrons are initially hot and then slow down by Cou-lomb losses, ionization, and excitation. Once they are coolenough (less than a few hundred eV), the positrons can undergocharge exchange with neutrals, recombine radiatively with freeelectrons, and/or annihilate directly with free or bound elec-trons, producing a line at 511 keV and a positronium continuum(Bussard et al. 1979; Guessoum et al. 2005). The fractions ofphotons that are emitted in the line and in the continuum dependon the properties of the annihilating medium. Recent analysisof data from SPI (the Spectrometer aboard INTEGRAL) yieldspositronium fractions close to 0.95 for the diffuse Galacticannihilation radiation (Churazov et al. 2005; Jean et al. 2006).For the width of the line, SPI finds 2.4 0.3 keV FWHM, ifthe line is approximated as a simple Gaussian (Churazov et al.2005).6 Note that the positronium fraction and the FWHM val-ues for the Galaxy are not necessarily appropriate for individualSN annihilations. The positronium fraction depends on the tem-perature and the ionization state of the medium (Guessoum etal. 1991; Jean et al. 2006), and the FWHM can be broadenedby Doppler effects if the annihilation takes place in the ejecta.For weak or radially combed magnetic fields that permit5% escape, positron-transport simulations estimate that roughly8#1052 positrons escape from a given Type Ia SN (Milne etal. 1999). When combined with the rate of 0.5 Type Ia SNe6 Recent analysis of the SPI observations of the Galactic center regionsuggest that the line may be composed of both a narrow (FWHM ∼ 1.3 keV)and a broad (FWHM ∼ 5.1 keV) component (Jean et al. 2006).L56 KALEMCI˙ ET AL. Vol. 640Fig. 1.—SPI significance image in the 508.5–513.5 keV band of the fullycoded field of view. Contours of SN 1006 from the Advanced Satellite forCosmology and Astrophysics are overlaid to indicate the extent and positionof the source. The 3 FWHM of the imaging resolution of SPI is shown forcomparison. The significance scale is shown at right.TABLE 1SPI Upper Limits511 keV Line FWHM(keV)Set I (250 ks)FluxSet II (750 ks)FluxCombinedFlux2.5 . . . . . . . . . . . . . . . . . . . . . . . 1.15#104 0.70#104 0.59#1043.5 . . . . . . . . . . . . . . . . . . . . . . . 1.35#104 0.81#104 0.70#1045.0 . . . . . . . . . . . . . . . . . . . . . . . 1.66#104 0.96#104 0.83#104Positronium Continuum350–500 keV band . . . . . . 5.8#104 3.4#104 3.0#104Note.—All fluxes are 3 j upper limits, photons cm2 s1.per century, a steady state flux of 9#104 photons cm2 s1is expected if the positrons are assumed to be annihilated at adistance of 8 kpc (the distance to the Galactic center). Thelatest measurements from SPI yield a total Galactic 511 keVflux of (1.5–2.9)#103 photons cm2 s1 depending on theGalactic distribution model (Kno¨dlseder et al. 2005). Thehigher end of that flux range is in agreement with observationsof the 511 keV line emission by the Oriented Scintillation Spec-trometer Experiment (OSSE) on the Compton Gamma Ray Ob-servatory, the Solar Maximum Mission, and the TransientGamma-Ray Spectrometer (Milne et al. 2001a). The expectedflux from the 5% escape case is therefore 30%–60% of thetotal flux, with the higher fluxes producing the lower SN frac-tions. Therefore, if positrons escape the ejecta but are confinedto local regions of the SNe, the 511 keV maps should trace therecent SN Ia history. The bulge-to-disk flux ratio is confinedto be within the range 1–3 (and luminosity ratio of 3–9). Thisratio is consistent with an absence of Galactic-scale diffusionand makes Type Ia SNe prime candidates for the source ofGalactic bulge positrons, along with low-mass X-ray binaries(see Kno¨dlseder et al. [2005] for a thorough discussion ofpotential candidates for diffuse annihilation radiation).The most promising individual source candidate to searchfor a positron annihilation line is SN 1006, a recent SN thoughtto be of Type Ia. Our group observed SN 1006 with INTEGRALfor ∼1000 ks, with the main goals of characterizing the hardX-ray emission using the ISGRI and JEM-X instruments (Ka-lemci et al. 2006) and searching for the 511 keV emission linewith SPI. In this Letter, we discuss the results of the analysisof the SPI data and place limits on the 511 keV line and posi-tronium continuum emission from SN 1006.2. OBSERVATIONS, ANALYSIS, AND RESULTSSPI is a coded-aperture telescope using an array of 19 cooledgermanium detectors for high-resolution spectroscopy (Ve-drenne et al. 2003). It works in the 20 keV to 8 MeV bandand has an energy resolution of ∼2 keV at 511 keV. The fullycoded field of view is 16, and the angular resolution is 3.The INTEGRAL observations of SN 1006 took place in twosets. The first, 250 ks set was conducted early in the mission,between MJD 52,650 and MJD 52,659, corresponding to IN-TEGRAL revolutions 30 and 32. These observations will bereferred to as “set I.” The second, 750 ks set was conductedbetween MJD 53,024 and MJD 53,034 during revolutions 155–158 and will be denoted as “set II.” For SPI, set I has all 19detectors working, whereas for set II the active detectors werereduced to 18 after the loss of detector 2.Before any analysis, we filtered out the pointings with highanticoincidence-shield rates, mostly occurring during the entryand exit of the radiation belts. We used OSA 4.2, SPIROS 6.1,and single-detector events for the analysis. Several backgroundmodels were tried with “GEDSAT” as the main model, whichassumes that the background level is proportional to the productof saturated detector trigger rate and live time. We used thecontinuum energy band at 523–545 keV for normalization. Wetried normalization with an OFF observation (empty-fieldobservation in revolution 130), and also using a template file(provided by J. Kno¨dlseder). The details of the backgroundmethods and information about the template file can be foundin Kno¨dlseder et al. (2005). Finally, we tried the mean countmodulation method in SPIROS, which does not require a priorbackground determination. The maximum likelihood methodin SPIROS was used for imaging. Note that SN 1006 is ∼30in diameter and is effectively a point source for SPI. Sets I andII were analyzed separately. We used 5, 7, and 10 keV energyintervals encompassing 511 keV to factor in the possibilitiesof both narrow and broad emission. We also searched for thepositronium continuum in the 200–500 keV band with SPI.Figure 1 shows the SPIROS sigma image in the 508.5–513.5 keV energy band, using the mean count modulationmethod. We obtained similar images in wider energy bands,and with different background methods. We have not detectedsignificant 511 keV line emission in any of the energy-bandintervals around 511 keV. The positronium component was notdetected either. Table 1 shows the details of the search and3 j sensitivity limits of SPI for the given energy band andobserving time. The combined upper limit is for 18 detectorsin sets I and II.3. DISCUSSIONSN 1006 is an ideal candidate to search for 511 keV positronannihilation line emission, as it is a well-studied, historical TypeIa SN with relatively well determined characteristics. The dis-tance is ∼2.2 kpc, the age is 999 yr, the angular size is ∼30,and the ejecta’s current electron density is 0.7 cm3 (Winkleret al. 2003b; Long et al. 2003; Milne 1971; Milne et al. 1999;Reynolds 1998).Predictions of the current 511 keV line and positronium con-tinuum fluxes from SN 1006 depend upon understanding thecomplicated interaction of the SN shock with the ISM and theresulting degree of magnetic confinement of positrons (Ruiz-Lapuente & Spruit 1998). Simulations have shown that posi-trons that escape from the SN ejecta typically have ∼500 keVNo. 1, 2006 ANNIHILATION RADIATION FROM SN 1006 L57Fig. 2.—Predicted 511 keV line fluxes from SN 1006 as a function ofpositron mean lifetime. The solid and dashed curves are for 5% and 12%escape fractions, respectively. The expected SPI (5 j) sensitivity to the511 keV line for 3500 ks and the current SPI 3 j upper limits for the emissionfrom the SNRs are shown for reference.of energy upon their escape (Chan & Lingenfelter 1993; Milneet al. 1999). The range of 500 keV positrons in the ISM issuch that few positrons would be expected to thermalize within1000 yr (Guessoum et al. 1991). This situation is even worseat the location of SN 1006, where the ISM density appearseven lower than four media treated by Guessoum et al. (1991).Thus, if the shock fails to confine escaping positrons, perhapseven accelerating the positrons, then SN 1006 would be onlya faint source of annihilation radiation, even if positrons es-caped the ejecta. The primary arguments against positronscrossing the shock in this fashion are circumstantial, as thedistribution of annihilation radiation from SNe would then tracethe distribution of matter in the Galaxy. It has been shown thatthe results of diffuse 511 keV emission mapping from OSSEand SPI (Purcell et al. 1997; Milne et al. 2001b; Kno¨dlseder etal. 2005; Teegarden et al. 2005) indicate a strong bulge compo-nent, with a bulge-to-disk luminosity ratio of 3–9. By contrast,the matter content of the Galaxy is concentrated in the disk(bulge-to-disk mass ratio of only 0.3–1.0; Robin et al. 2003),which appears to eliminate large-scale positron transport fromsource to annihilation site (not just from SNe, but in general).If the magnetic field in the shock successfully confines thepositrons, they will repeatedly traverse the inside of the bubble,being reflected numerous times. Preliminary simulations ofthis scenario suggest that the positrons will encounter a suf-ficient amount of matter to be thermalized and annihilate on a1000 yr timescale. However, these simulations are too crudefor us to confidently predict an annihilation rate at 1000 yearsafter the SN event. A more detailed simulation will be con-ducted for analysis of the entire data set, including this andobservations in the near future.In Figure 2, we show the predicted 511 keV fluxes from SN1006 as a function of mean positron lifetime for the 5% escapefraction. The figure also shows the current 3 j SPI upper limitand the 3 j sensitivity level that will be achieved after theapproved future observations (2.5 Ms). If 5% of the positronsindeed escape the ejecta and are locally confined with a lifetimeof less than 105 yr, then SPI is expected to detect 511 keVannihilation emission from SN 1006 once the additionalobservations are performed.Figure 2 also shows the predicted fluxes for SN 1006 if thepositron yield is increased by a factor of 2.5, so that SNe Iacould account for all Galactic positrons. The SPI nondetectionsuggests against this possibility if the mean positron lifetimeis less than 105 yr. Note that in the case of no escape from theremnant, the expected 511 keV flux from only 44Ti decays isno larger than 4#106 photons cm2 s1 from SN 1006, whichis far too low to be detectable by SPI.We stress that the conclusions above depend strongly on theassumption of no escape to the ISM and thermalization in theejecta. The future observations of SN 1006, and other SN Iaremnants of different ages (both younger and older), will provideessential information for understanding positron transport inSNRs. A nondetection would still leave an ambiguity; the reasoncould be either ∼0% escape fraction from the ejecta or a verylong thermalization timescale for positrons in the ISM. On theother hand, detection of annihilation radiation would be a strongindicator of thermalization in the ejecta and confirm whetherSNe Ia are important contributors of Galactic positrons.E. K. is supported by the European Commission through theFP6 Marie Curie International Reintegration Grant (INDAM)and also acknowledges partial support from TU¨ BI˙TAK. E. K.and S. E. B. acknowledge NASA grants NAG 5-13142 andNAG 5-13093. S. P. R. acknowledges support from NASAgrant NAG 5-13092. E. K. thanks Pierre Jean for useful dis-cussions and also thanks the reviewer, Richard Lingenfelter,for remarks that significantly improved this Letter.REFERENCESBussard, R. W., Ramaty, R., & Drachman, R. J. 1979, ApJ, 228, 928Chan, K.-W., & Lingenfelter, R. E. 1993, ApJ, 405, 614Churazov, E., Sunyaev, R., Sazonov, S., Revnivtsev, M., & Varshalovich, D.2005, MNRAS, 357, 1377Guessoum, N., Jean, P., & Gillard, W. 2005, A&A, 436, 171Guessoum, N., Ramaty, R., & Lingenfelter, R. E. 1991, ApJ, 378, 170Jean, P., Kno¨dlseder, J., Gillard, W., Guessoum, N., Ferrie`re, K., Marcowith,A., Lonjou, V., & Roques, J.-P. 2006, A&A, 445, 579Kalemci, E., Reynolds, S. P., Boggs, S. E., Lund, N., Chenevez, J., Renaud,M., & Rho, J. 2006, ApJ, in press (astro-ph/0602335)Kno¨dlseder, J., et al. 2005, A&A, 441, 513Long, K. S., Reynolds, S. P., Raymond, J. C., Winkler, P. F., Dyer, K. K., &Petre, R. 2003, ApJ, 586, 1162Milne, D. K. 1971, Australian J. Phys., 24, 757Milne, P. A., Kurfess, J. D., Kinzer, R. L., Leising, M. D., & Dixon, D. D.2001a, in Exploring the Gamma-Ray Universe, ed. A. Gimenez, V. Reglero,& C. Winkler (ESA SP-459) (Noordwijk: ESA), 145Milne, P. A., The, L.-S., & Leising, M. D. 1999, ApJS, 124, 503———. 2001b, ApJ, 559, 1019Purcell, W. R., et al. 1997, ApJ, 491, 725Reynolds, S. P. 1998, ApJ, 493, 375Robin, A. C., Reyle´, C., Derrie`re, S., & Picaud, S. 2003, A&A, 409, 523(erratum 416, 157 [2004])Ruiz-Lapuente, P., & Spruit, H. C. 1998, ApJ, 500, 360Sollerman, J., et al. 2004, A&A, 428, 555Spyromilio, J., Gilmozzi, R., Sollerman, J., Leibundgut, B., Fransson, C., &Cuby, J.-G. 2004, A&A, 426, 547Teegarden, B. J., et al. 2005, ApJ, 621, 296Vedrenne, G., et al. 2003, A&A, 411, L63Winkler, C., et al. 2003a, A&A, 411, L1Winkler, P. F., Gupta, G., & Long, K. S. 2003b, ApJ, 585, 324

Searching for annihilation radiation from SN 1006 with SPI on INTEGRAL

Crossref

Searching for Annihilation Radiation from SN 1006 with SPI on
                    INTEGRAL

http://research.sabanciuniv.edu/6445/1/sn1006_511_ek.pdf

Searching for annihilation radiation from SN 1006 with SPI on INTEGRAL

Abstract

Similar works

Full text

Available Versions

Sabanci University Research Database

Crossref